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Found 185 entries in the Bibliography.

Showing entries from 101 through 150

2018

Response of Different Ion Species to Local Magnetic Dipolarization Inside Geosynchronous Orbit

This paper examines how hydrogen, helium and oxygen (H, He and O) ion fluxes at 1\textendash1000 keV typically respond to local magnetic dipolarization inside geosynchronous orbit (GEO). We extracted 144 dipolarizations which occurred at magnetic inclination > 30\textdegree from the 2012\textendash2016 tail seasons\textquoteright observations of the Van Allen Probes spacecraft and then defined typical flux changes of these ion species by performing a superposed epoch analysis. On average, the dipolarization inside GEO is accompanied by a precursory transient decrease in the northward magnetic field component, transient impulsive enhancement in the westward electric field component, and decrease (increase) in the proton density (temperature). The coincident ion species experience an energy-dependent flux change, consisting of enhancement (depression) at energies above (below) ~50 keV. These properties morphologically resemble those around dipolarization fronts (or fast flows) in the near-Earth tail. A distinction among the ion species is the average energy of the flux ratio peak, being at 200\textendash400 keV (100\textendash200 keV) for He (H and O) ions. The flux ratio peaks at different energies likely reflect the different charge states of injected ionospheric- and/or solar wind-origin ion species. The ion spectra become harder for sharp dipolarizations, suggesting the importance of accompanying electric field in transporting and/or energizing the ions efficiently. Interestingly, the average flux ratio peak does not differ significantly among the ion species for ~2 min after onset, which implies that mass-dependent acceleration process is less important in the initial stage of dipolarization.

Motoba, T.; Ohtani, S.; Gkioulidou, M.; Ukhorskiy, A.; Mitchell, D.; Takahashi, K.; Lanzerotti, L.; Kletzing, C.; Spence, H.; Wygant, J.;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018

YEAR: 2018 DOI: 10.1029/2018JA025557

deep inside geosynchronous orbit; dipolarizations; Ion injections; ion species; Van Allen Probes

Spatial Development of the Dipolarization Region in the Inner Magnetosphere

The present study examines dipolarization events observed by the Van Allen Probes within 5.8 RE from Earth. It is found that the probability of occurrence is significantly higher in the dusk-to-midnight sector than in the midnight-to-dawn sector, and it deceases sharply earthward. A comparison with observations made at nearby satellites shows that dipolarization signatures are often highly correlated (c.c. > 0.8) within 1 hr in MLT and 1 RE in RXY, and the dipolarization region expands earthward and westward in the dusk-to-midnight sector. The westward expansion velocity is estimated at 0.4 hr (in MLT) per minute, or 60 km/s, which is consistent with the previously reported result for geosynchronous dipolarization. The earthward expansion is apparently less systematic than the westward expansion. Its velocity is estimated at 50 km/s (0.5 RE/min), comparable to the westward expansion velocity, but it is suggested that the earthward expansion slows down as the dipolarization region approaches Earth, and it eventually stops. This idea is consistent with the earthward reduction of the occurrence probability of dipolarization events. Whereas this earthward expansion is difficult to explain with the conventional wedge current system, it may be understood in terms of a current system with two wedges, one with the R1 polarity outside and the other with the R2 polarity closer to Earth. For such a current system the region of dipolarization is confined in radial distance between the two wedge currents, and it is considered to expand earthward as the R2-sense wedge moves earthward along with injected plasma.

Ohtani, S.; Motoba, T.; Gkioulidou, M.; Takahashi, K.; Singer, H.;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018

YEAR: 2018 DOI: 10.1029/2018JA025443

Dipolarization; injection; inner magnetosphere; R1 and R2 currents; substorm current wedge; substorms; Van Allen Probes

Poloidal mode wave-particle interactions inferred from Van Allen Probes and CARISMA ground-based observations

Ultra-low-frequency (ULF) wave and test particle models are used to investigate the pitch angle and energy dependence of ion differential fluxes measured by the Van Allen Probes spacecraft on October 6th, 2012. Analysis of the satellite data reveals modulations in differential flux resulting from drift resonance between H+ ions and fundamental mode poloidal Alfv\ en waves detected near the magnetic equator at L\~5.7. Results obtained from simulations reproduce important features of the observations, including a substantial enhancement of the differential flux between \~20\textdegree - 40\textdegree pitch angle for ion energies between \~90 - 220keV, and an absence of flux modulations at 90\textdegree. The numerical results confirm predictions of drift-bounce resonance theory and show good quantitative agreement with observations of modulations in differential flux produced by ULF waves.

Wang, C.; Rankin, R.; Wang, Y.; Zong, Q.-G.; Zhou, X.; Takahashi, K.; Marchand, R.; Degeling, A.;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2018

YEAR: 2018 DOI: 10.1029/2017JA025123

ULF wave; drift-resonant; test particle simulation; Van Allen Probes

Van Allen Probes Observation of a Fundamental Poloidal Standing Alfv\ en Wave Event Related to Giant Pulsations

The Van Allen Probes-A spacecraft observed an \~9 mHz ultra-low-frequency wave on 6 October 2012, at L\~ 5.7, in the dawn sector, and very near the magnetic equator. The wave had a strong electric field that was initially stronger in the azimuthal component and later in the radial component, exhibited properties of a fundamental standing Alfv\ en wave, and was associated with giant pulsations observed on the ground near the magnetic field footprint of the spacecraft. The wave was accompanied by oscillations of the flux of energetic protons (jH+). The amplitude of urn:x-wiley:jgra:media:jgra54254:jgra54254-math-0001 oscillations was large at equatorial pitch angles away from 90\textdegree, and the energy dependence of the phase and amplitude of the oscillations exhibited features consistent with drift resonance of \~140 keV protons with a westward-propagating wave having an azimuthal wave number of \~-40. The wave was detected when the spacecraft entered a region of an earthward gradient of the proton phase space density, in support of a theoretical prediction that such a gradient can drive fundamental poloidal waves.

Takahashi, Kazue; Claudepierre, S.; Rankin, Robert; Mann, Ian; Smith, C.;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2018

YEAR: 2018 DOI: 10.1029/2017JA025139

drift resonance; Fundamental standing Alfven wave; Giant pulsation; Proton flux oscillation; Van Allen Probes

Van Allen Probes observations of drift-bounce resonance and energy transfer between energetic ring current protons and poloidal Pc4 wave

A poloidal Pc4 wave and proton flux oscillations are observed in the inner magnetosphere on the dayside near the magnetic equator by the Van Allen Probes spacecraft on 2 March 2014. The flux oscillations are observed in the energy range of 67.0 keV to 268.8 keV with the same frequency of the poloidal Pc4 wave. We find pitch angle and energy dispersion in the phase difference between the poloidal magnetic field and the proton flux oscillations, which are features of drift-bounce resonance. We estimate the resonance energy to be ~120 keV for pitch angle (α) of 30\textdegree or 150\textdegree, and 170\textendash180 keV for α = 50\textdegree or 130\textdegree. To examine the direction of energy flow between protons and the wave, we calculate the sign of the gradient of proton phase space density (df/dW) on both the inbound and outbound legs of the orbit. We find the gradient to be outward on both legs, which means that energy is transferred from the protons to the wave. During the poloidal Pc4 wave event, the Dst* index shows a measurable increase of ~6.7 nT. We estimate the total energy loss of the ring current from the recovery of the Dst* index and from the variation of proton flux by the drift-bounce resonance. We suggest that energy transfer from the ring current protons to the poloidal Pc4 wave via the drift-bounce resonance contributes to up to ~85 \% of the increase of the Dst* index.

Oimatsu, S.; e, M.; Takahashi, K.; Yamamoto, K.; Keika, K.; Kletzing, C.; Smith, C.; MacDowall, R.; Mitchell, D.;

Published by: Journal of Geophysical Research: Space Physics Published on: 04/2018

YEAR: 2018 DOI: 10.1029/2017JA025087

Van Allen Probes

Observation and Numerical Simulation of Cavity Mode Oscillations Excited by an Interplanetary Shock

Cavity mode oscillations (CMOs) are basic magnetohydrodynamic eigenmodes in the magnetosphere predicted by theory and are expected to occur following the arrival of an interplanetary shock. However, observational studies of shock-induced CMOs have been sparse. We present a case study of a dayside ultra-low-frequency (ULF) wave event that exhibited CMO properties. The event occurred immediately following the arrival of an interplanetary shock at 0829 UT on 15 August 2015. The shock was observed in the solar wind by the Time History of Events and Macroscale Interactions during Substorms-B and -C spacecraft, and magnetospheric ULF waves were observed by multiple spacecraft including the Van Allen Probes-A and -B spacecraft, which were located in the dayside plasmasphere at L\~ 1.4 and L\~ 2.4, respectively. Both Van Allen Probes spacecraft detected compressional poloidal mode oscillations at \~13 mHz (fundamental) and \~26 mHz (second harmonic). At both frequencies, the azimuthal component of the electric field (Eϕ) lagged behind the compressional component of the magnetic field (Bμ) by \~90o. The frequencies and the Eϕ-Bμ relative phase are in good agreement with the CMOs generated in a dipole magnetohydrodynamic simulation that incorporates a realistic plasma mass density distribution and ionospheric boundary condition. The oscillations were also detected on the ground by the European quasi-Meridional Magnetometer Array, which was located near the magnetic field footprints of the Van Allen Probes spacecraft.

Takahashi, Kazue; Lysak, Robert; Vellante, Massimo; Kletzing, Craig; Hartinger, Michael; Smith, Charles;

Published by: Journal of Geophysical Research: Space Physics Published on: 02/2018

YEAR: 2018 DOI: 10.1002/2017JA024639

Cavity mode oscillations; interplanetary shock; Van Allen Probes

Three-Step Buildup of the 17 March 2015 Storm Ring Current: Implication for the Cause of the Unexpected Storm Intensification

We examine the spatiotemporal variations of the energy density and the energy spectral evolution of energetic ions in the inner magnetosphere during the main phase of the 17 March 2015 storm, using data from the RBSPICE and EMFISIS instruments onboard Van Allen Probes. The storm developed in response to two southward IMF intervals separated by about 3 h. In contrast to two steps seen in the Dst/SYM-H index, the ring current ion population evolved in three steps: the first subphase was apparently caused by the earlier southward IMF, and the subsequent subphases occurred during the later southward IMF period. Ion energy ranges that contribute to the ring current differed between the three subphases. We suggest that the spectral evolution resulted from the penetration of different plasma sheet populations. The ring current buildup during the first subphase was caused by the penetration of a relatively low-energy population that had existed in the plasma sheet during a prolonged prestorm northward IMF interval. The deeper penetration of the lower-energy population was responsible for the second subphase. The third subphase, where the storm was unexpectedly intensified to a Dst/SYM-H level of <-200 nT, was caused by the penetration of a hot, dense plasma sheet population. We attribute the hot, dense population to the entry of hot, dense solar wind into the plasma sheet and/or ion heating/acceleration in the near-Earth plasma sheet associated with magnetotail activity such as reconnection and dipolarization.

Keika, Kunihiro; Seki, Kanako; e, Masahito; Miyoshi, Yoshizumi; Lanzerotti, Louis; Mitchell, Donald; Gkioulidou, Matina; Manweiler, Jerry;

Published by: Journal of Geophysical Research: Space Physics Published on: 01/2018

YEAR: 2018 DOI: 10.1002/2017JA024462

enhancements of oxygen ions of ionospheric origin; plasma transport from the plasma sheet into the inner magnetosphere; RBSPICE; unexpected intensification of the magnetic storm; Van Allen Probes

Van Allen Probes Observations of Second Harmonic Poloidal Standing Alfv\ en Waves

Long-lasting second-harmonic poloidal standing Alfv\ en waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (\~10 mHz) were also observed in the energy (W) range 50\textendash300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of \~-200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180\textdegree (southward moving protons) at W \~ 120 keV. This feature is explained by drift-bounce resonance (mωd \~ ωb) of \~120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density ( math formula) exhibits a bump-on-tail structure with math formula occurring in the 1\textendash10 keV energy range. This math formula is unstable and can excite P2 waves through bounce resonance (ω \~ ωb), where ω is the wave frequency.

Takahashi, Kazue; Oimatsu, Satoshi; e, Masahito; Min, Kyungguk; Claudepierre, Seth; Chan, Anthony; Wygant, John; Kim, Hyomin;

Published by: Journal of Geophysical Research: Space Physics Published on: 01/2018

YEAR: 2018 DOI: 10.1002/2017JA024869

bounce and drift-bounce resonances; energetic protons; plasmasphere; poloidal ULF waves; second harmonic; Van Allen Probes

2017

SC-associated electric field variations in the magnetosphere and ionospheric convective flows

We examine magnetic and electric field perturbations associated with a sudden commencement (SC), caused by an interplanetary (IP) shock passing over the Earth\textquoterights magnetosphere on 16 February 2013. The SC was identified in the magnetic and electric field data measured at THEMIS-E (THE-E: MLT = 12.4, L = 6.3), Van Allen Probe-A (VAP-A: MLT = 3.2, L = 5.1), and Van Allen Probe-B (VAP-B: MLT = 0.2. L= 4.9) in the magnetosphere. During the SC interval, THE-E observed a dawnward-then-duskward electric (E) field perturbation around noon, while VAP-B observed a duskward E-field perturbation around midnight. VAP-A observed a dawnward-then-duskward E-field perturbation in the postmidnight sector, but the duration and magnitude of the dawnward E-perturbation are much shorter and weaker than that at THE-E. That is, the E-field signature changes with local time during the SC interval. The SuperDARN radar data indicate that the ionospheric plasma motions during the SC are mainly due to the E-field variations observed in space. This indicates that the SC-associated E-field in space plays a significant role in determining the dynamic variations of the ionospheric convection flow. By comparing previous SC MHD simulations and our observations, we suggest that the E-field variations observed at the spacecraft are produced by magnetospheric convection flows due to deformation of the magnetosphere as the IP shock sweeps the magnetopause.

Kim, S.-I.; Kim, K.-H.; Kwon, H.-J.; Jin, H.; Lee, E.; Jee, G.; Nishitani, N.; Hori, T.; Lester, M.; Wygant, J.;

Published by: Journal of Geophysical Research: Space Physics Published on: 10/2017

YEAR: 2017 DOI: 10.1002/2017JA024611

electric field; Sudden commencement; Van Allen Probes

Energetic proton spectra measured by the Van Allen Probes

We test the hypothesis that pitch-angle scattering by electromagnetic ion cyclotron (EMIC) waves can limit ring current proton fluxes. For two chosen magnetic storms, during March 17-20, 2013 and March 17-20, 2015, we measure proton energy spectra in the region 3 <= L <= 6 using the RBSPICE B instrument on the Van Allen Probes. The most intense proton spectra are observed to occur during the recovery periods of the respective storms. Using proton precipitation data from the POES (NOAA and MetOp) spacecraft, we deduce that EMIC wave action was prevalent at the times and L-shell locations of the most intense proton spectra. We calculate limiting ring current proton energy spectra from recently developed theory. Comparisons between the observed proton energy spectra and the theoretical limiting spectra show reasonable agreement. We conclude that the measurements of the most intense proton spectra are consistent with self-limiting by EMIC wave scattering.

Summers, Danny; Shi, Run; Engebretson, Mark; Oksavik, Kjellmar; Manweiler, Jerry; Mitchell, Donald;

Published by: Journal of Geophysical Research: Space Physics Published on: 09/2017

YEAR: 2017 DOI: 10.1002/2017JA024484

EMIC-wave -proton scattering; proton ring current; Van Allen Probes

Low-energy (< 200 eV) electron acceleration by ULF waves in the plasmaspheric boundary layer: Van Allen Probes observation

We report observational evidence of cold plamsmaspheric electron (< 200 eV) acceleration by ultra-low-frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 minute which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10-20 keV) bi-directional pitch angle signatures observed during the event are suggestive of the drift-bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15 and MMS 1 observations suggest ULF waves in the event were strongest on the dusk-side magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump-on-tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift-bounce resonant instability, with the free energy supplied by substorm-injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere.

Ren, Jie; Zong, Q.; Miyoshi, Y.; Zhou, X.; Wang, Y.; Rankin, R.; Yue, C.; Spence, H.; Funsten, H.; Wygant, J.; Kletzing, C.;

Published by: Journal of Geophysical Research: Space Physics Published on: 08/2017

YEAR: 2017 DOI: 10.1002/2017JA024316

Cold plasmaspheric electrons; drift-bounce resonance; Plasma instability; Plasmaspheric boundary layer; Substorm-injected protons; ULF waves; Van Allen Probes

The characteristic pitch angle distributions of 1 eV to 600 keV protons near the equator based on Van Allen Probes observations

Understanding the source and loss processes of various plasma populations is greatly aided by having accurate knowledge of their pitch angle distributions (PADs). Here, we statistically analyze ~1 eV to 600 keV hydrogen (H+) PADs near the geomagnetic equator in the inner magnetosphere based on Van Allen Probes measurements, to comprehensively investigate how the H+ PADs vary with different energies, magnetic local times (MLTs), L-shells, and geomagnetic conditions. Our survey clearly indicates four distinct populations with different PADs: (1) a pancake distribution of the plasmaspheric H+ at low L-shells except for dawn sector; (2) a bi-directional field-aligned distribution of the warm plasma cloak; (3) pancake or isotropic distributions of ring current H+; (4) radiation belt particles show pancake, butterfly and isotropic distributions depending on their energy, MLT and L-shell. Meanwhile, the pancake distribution of ring current H+ moves to lower energies as L-shell increases which is primarily caused by adiabatic transport. Furthermore, energetic H+ (> 10 keV) PADs become more isotropic following the substorm injections, indicating wave-particle interactions. The radiation belt H+ butterfly distributions are identified in a narrow energy range of 100 < E < 400 keV at large L (L > 5), which are less significant during quiet times and extend from dusk to dawn sector through midnight during substorms. The different PADs near the equator provide clues of the underlying physical processes that produce the dynamics of these different populations.

Yue, Chao; Bortnik, Jacob; Thorne, Richard; Ma, Qianli; An, Xin; Chappell, C.; Gerrard, Andrew; Lanzerotti, Louis; Shi, Quanqi; Reeves, Geoffrey; Spence, Harlan; Mitchell, Donald; Gkioulidou, Matina; Kletzing, Craig;

Published by: Journal of Geophysical Research: Space Physics Published on: 08/2017

YEAR: 2017 DOI: 10.1002/2017JA024421

bi-directional field-aligned; H+ Pitch angle distributions; plasmaspheric H+; radiation belt H+; ring current; Van Allen Probes; warm Plasma cloak

Statistical study of the storm-time radiation belt evolution during Van Allen Probes era: CME- versus CIR-driven storms

CME- or CIR-driven storms can change the electron distributions in the radiation belt dramatically, which can in turn affect the spacecraft in this region or induce geomagnetic effects. The Van Allen Probes twin spacecraft, launched on 30 August 2012, orbit near the equatorial plane and across a wide range of L* with apogee at 5.8 RE and perigee at 620 km. Electron data from Van Allen Probes MagEIS and REPT instruments have been binned every six hours at L*=3 (defined as 2.5

Shen, Xiao-Chen; Hudson, Mary; Jaynes, Allison; Shi, Quanqi; Tian, Anmin; Claudepierre, Seth; Qin, Mu-Rong; Zong, Qiu-Gang; Sun, Wei-Jie;

Published by: Journal of Geophysical Research: Space Physics Published on: 07/2017

YEAR: 2017 DOI: 10.1002/2017JA024100

CIR-driven storm; CME-driven storm; outer radiation belt; Van Allen Probes

Roles of hot electrons in generating upper-hybrid waves in the earth\textquoterights radiation belt

Electrostatic fluctuations near upper-hybrid frequency, which are sometimes accompanied by multiple-harmonic electron cyclotron frequency bands above and below the upper-hybrid frequency, are common occurrences in the Earth\textquoterights radiation belt, as revealed through the twin Van Allen Probe spacecrafts. It is customary to use the upper-hybrid emissions for estimating the background electron density, which in turn can be used to determine the plasmapause locations, but the role of hot electrons in generating such fluctuations has not been discussed in detail. The present paper carries out detailed analyses of data from the Waves instrument, which is part of the Electric and Magnetic Field Instrument Suite and Integrated Science suite onboard the Van Allen Probes. Combined with the theoretical calculation, it is shown that the peak intensity associated with the upper-hybrid fluctuations might be predominantly determined by tenuous but hot electrons and that denser cold background electrons do not seem to contribute much to the peak intensity. This finding shows that upper-hybrid fluctuations detected during quiet time are not only useful for the determination of the background cold electron density but also contain information on the ambient hot electrons population as well.

Hwang, J.; Shin, D.; Yoon, P.; Kurth, W.; Larsen, B.; Reeves, G.; Lee, D;

Published by: Physics of Plasmas Published on: 06/2017

YEAR: 2017 DOI: 10.1063/1.4984249

Hot carriers; Magnetized plasmas; Radiation belts; Singing; Van Allen Probes; Whistler waves

Energetic electron precipitation and auroral morphology at the substorm recovery phase

It is well known that auroral patterns at the substorm recovery phase are characterized by diffuse or patch structures with intensity pulsation. According to satellite measurements and simulation studies, the precipitating electrons associated with these aurorae can reach or exceed energies of a few hundreds of keV through resonant wave-particle interactions in the magnetosphere. However, because of difficulty of simultaneous measurements, the dependency of energetic electron precipitation (EEP) on auroral morphological changes in the mesoscale has not been investigated to date. In order to study this dependency, we have analyzed data from the European Incoherent Scatter (EISCAT) radar, the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) riometer, collocated cameras, ground-based magnetometers, the Van Allen Probe satellites, Polar Operational Environmental Satellites (POES), and the Antarctic-Arctic Radiation-belt (Dynamic) Deposition-VLF Atmospheric Research Konsortium (AARDDVARK). Here we undertake a detailed examination of two case studies. The selected two events suggest that the highest energy of EEP on those days occurred with auroral patch formation from postmidnight to dawn, coinciding with the substorm onset at local midnight. Measurements of the EISCAT radar showed ionization as low as 65 km altitude, corresponding to EEP with energies of about 500 keV.

Oyama, S.; Kero, A.; Rodger, C.; Clilverd, M.; Miyoshi, Y.; Partamies, N.; Turunen, E.; Raita, T.; Verronen, P.; Saito, S.;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2017

YEAR: 2017 DOI: 10.1002/2016JA023484

auroral patch; EEP; Ionosphere; plasma wave; recovery phase; substorm; Van Allen Probes

Global observations of magnetospheric high- m poloidal waves during the 22 June 2015 magnetic storm

We report global observations of high-m poloidal waves during the recovery phase of the 22 June 2015 magnetic storm from a constellation of widely spaced satellites of five missions including Magnetospheric Multiscale (MMS), Van Allen Probes, Time History of Events and Macroscale Interactions during Substorm (THEMIS), Cluster, and Geostationary Operational Environmental Satellites (GOES). The combined observations demonstrate the global spatial extent of storm time poloidal waves. MMS observations confirm high azimuthal wave numbers (m ~ 100). Mode identification indicates the waves are associated with the second harmonic of field line resonances. The wave frequencies exhibit a decreasing trend as L increases, distinguishing them from the single-frequency global poloidal modes normally observed during quiet times. Detailed examination of the instantaneous frequency reveals discrete spatial structures with step-like frequency changes along L. Each discrete L shell has a steady wave frequency and spans about 1 RE, suggesting that there exist a discrete number of drift-bounce resonance regions across L shells during storm times.

Le, G.; Chi, P.; Strangeway, R.; Russell, C.; Slavin, J.; Takahashi, K.; Singer, H.; Anderson, B.; Bromund, K.; Fischer, D.; Kepko, E.; Magnes, W.; Nakamura, R.; Plaschke, F.; Torbert, R.;

Published by: Geophysical Research Letters Published on: 04/2017

YEAR: 2017 DOI: 10.1002/2017GL073048

field line resonances; high-m poloidal waves; magnetic storm; magnetospheric multiscale mission; ULF waves; Van Allen Probes

Variations of the relativistic electron flux after a magnetospheric compression event

On January 21, 2015, a sharp increase of the solar wind dynamic pressure impacted the magnetosphere. The magnetopause moved inward to the region L< 8 without causing a geomagnetic storm. The flux of the relativistic electrons in the outer radiation belt decreased by half during this event based on the observations of the particle radiation monitor (PRM) of the fourth of the China-Brazil Earth Resource Satellites (CBERS-4). The flux remained low for approximately 11 d; it did not recover after a small magnetic storm on January 26 but after a small magnetic storm on February 2. The loss and recovery of the relativistic electrons during this event are investigated using the PRM data, medium- and high-energy electron observations of NOAA-15 and the Van Allen Probes, medium-energy electron observations of GOES-13, and wave observations of the Van Allen Probes. This study shows that the loss of energetic electrons in this event is related to magnetospheric compression. The chorus waves accelerate the medium-energy electrons, which causes the recovery of relativistic electrons. The Van Allen Probes detected strong chorus waves in the region L = 3\textendash6 from January 21 to February 2. However, the flux of medium-energy electrons was low in the region. This implies that the long-lasting lack of recovery of the relativistic electrons after this event is due to the lack of the medium-energy \textquotedblleftseed\textquotedblright electrons. The medium-energy electrons in the outer radiation belt may be a clue to predict the recovery of relativistic electrons.

Chen, Zhe; Chen, HongFei; Li, YiFan; Xiang, HongWen; Yu, XiangQian; Shi, WeiHong; Hao, ZhiHua; Zou, Hong; Zou, JiQing; Zhong, WeiYing;

Published by: Science China Technological Sciences Published on: 04/2017

YEAR: 2017 DOI: 10.1007/s11431-016-9008-3

outer radiation belt high-energy electrons medium-energy electrons space environment; Van Allen Probes

Inferring electromagnetic ion cyclotron wave intensity from low altitude POES proton flux measurements: A detailed case study with conjugate Van Allen Probes observations

Zhang, Yang; Shi, Run; Ni, Binbin; Gu, Xudong; Zhang, Xianguo; Zuo, Pingbing; Fu, Song; Xiang, Zheng; Wang, Qi; Cao, Xing; Zou, Zhengyang;

Published by: Advances in Space Research Published on: 03/2017

YEAR: 2017 DOI: 10.1016/j.asr.2016.12.035

Van Allen Probes

Oxygen cyclotron harmonic waves observed by the Van Allen Probes

Fine structured multiple-harmonic electromagnetic emissions at frequencies around the equatorial oxygen cyclotron harmonics are observed by Van Allen Probe A outside the core plasmasphere (L~5) off the magnetic equator (MLAT~-7.5\textdegree) during a magnetic storm. We find that the multiple-harmonic emissions have their PSD peaks at 2~8 equatorial oxygen gyro-harmonics (f~nfO+, n=2~8) while the fundamental mode (n=1) is absent, implying that the harmonic waves are generated near the equator and propagate into the observation region. Additionally these electromagnetic emissions are linear polarized. Different from the equatorial noise emission propagating very obliquely, these emissions have moderate wave normal angles (about 40\textdegree~60\textdegree) which predominately become larger as the harmonic number increases. Considering their frequency and wave normal angle characteristics, it is suggested that these multiple-harmonic emissions might play an important role in the dynamic variation of radiation belt electrons.

Xiongdong, Yu; Zhigang, Yuan; Dedong, Wang; Shiyong, Huang; Haimeng, Li; Tao, Yu; Zheng, Qiao;

Published by: Science China: Earth Sciences Published on: 03/2017

YEAR: 2017 DOI: 10.1007/s11430-016-9024-3

Oxygen Cyclotron Harmonic Waves; Radiation belt; Ring current ions; Van Allen Probes

Second harmonic poloidal waves observed by Van Allen Probes in the dusk-midnight sector

This paper presents observations of ultralow-frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single-spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be \~100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (\~0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy-dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift-bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift-bounce resonance mechanism with free energy fed by the inward radial gradient of \~80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Min, Kyungguk; Takahashi, Kazue; Ukhorskiy, Aleksandr; Manweiler, Jerry; Spence, Harlan; Singer, Howard; Claudepierre, Seth; Larsen, Brian; Soto-Chavez, Rualdo; Cohen, Ross;

Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017

YEAR: 2017 DOI: 10.1002/2016JA023770

drift-bounce resonance; high m ULF waves; Second harmonic poloidal mode; Van Allen Probes

Second harmonic poloidal waves observed by Van Allen Probes in the dusk-midnight sector

This paper presents observations of ultra-low frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred two days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the pre-midnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single-spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be \~100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (\~0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy-dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift-bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift-bounce resonance mechanism with free energy fed by the inward radial gradient of \~80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Min, Kyungguk; Takahashi, Kazue; Ukhorskiy, Aleksandr; Manweiler, Jerry; Spence, Harlan; Singer, Howard; Claudepierre, Seth; Larsen, Brian; Soto-Chavez, Rualdo; Cohen, Ross;

Published by: Journal of Geophysical Research: Space Physics Published on: 02/2017

YEAR: 2017 DOI: 10.1002/2016JA023770

drift-bounce resonance; high m ULF waves; Second harmonic poloidal mode; Van Allen Probes

Externally driven plasmaspheric ULF waves observed by the Van Allen Probes

We analyze data acquired by the Van Allen Probes on 8 November 2012, during a period of extended low geomagnetic activity, to gain new insight into plasmaspheric ultralow frequency (ULF) waves. The waves exhibited strong spectral power in the 5\textendash40 mHz band and included multiharmonic toroidal waves visible up to the eleventh harmonic, unprecedented in the plasmasphere. During this wave activity, the interplanetary magnetic field cone angle was small, suggesting that the waves were driven by broadband compressional ULF waves originating in the foreshock region. This source mechanism is supported by the tailward propagation of the compressional magnetic field perturbations at a phase velocity of a few hundred kilometers per second that is determined by the cross-phase analysis of data from the two spacecraft. We also find that the coherence and phase delay of the azimuthal components of the magnetic field from the two spacecraft strongly depend on the radial separation of the spacecraft and attribute this feature to field line resonance effects. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for L = 2.6\textendash5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we infer that the plasma was dominated by H+ ions and was distributed uniformly along the magnetic field lines. The electron density is higher than the prediction of saturated plasmasphere models, and this \textquotedblleftsuper saturated\textquotedblright plasmasphere and the uniform ion distribution are consistent with the low geomagnetic activity that prevailed.

Takahashi, Kazue; Denton, Richard; Kurth, William; Kletzing, Craig; Wygant, John; Bonnell, John; Dai, Lei; Min, Kyungguk; Smith, Charles; MacDowall, Robert;

Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017

YEAR: 2017 DOI: 10.1002/2014JA020373

multispacecraft observation; plasmasphere; ULF waves; Van Allen Probes

2016

Void structure of O ⁺ ions in the inner magnetosphere observed by the Van Allen Probes

The Van Allen Probes Helium Oxygen Proton Electron instrument observed a new type of enhancement of O+ ions in the inner magnetosphere during substorms. As the satellite moved outward in the premidnight sector, the flux of the O+ ions with energy ~10 keV appeared first in the energy-time spectrograms. Then, the enhancement of the flux spread toward high and low energies. The enhanced flux of the O+ ions with the highest energy remained, whereas the flux of the ions with lower energy vanished near apogee, forming what we call the void structure. The structure cannot be found in the H+ spectrogram. We studied the generation mechanism of this structure by using numerical simulation. We traced the trajectories of O+ ions in the electric and magnetic fields from the global magnetohydrodynamics simulation and calculated the flux of O+ ions in the inner magnetosphere in accordance with the Liouville theorem. The simulated spectrograms are well consistent with the ones observed by Van Allen Probes. We suggest the following processes. (1) When magnetic reconnection starts, an intensive equatorward and tailward plasma flow appears in the plasma lobe. (2) The flow transports plasma from the lobe to the plasma sheet where the radius of curvature of the magnetic field line is small. (3) The intensive dawn-dusk electric field transports the O+ ions earthward and accelerates them nonadiabatically to an energy threshold; (4) the void structure appears at energies below the threshold.

Nakayama, Y.; Ebihara, Y.; Ohtani, S.; Gkioulidou, M.; Takahashi, K.; Kistler, L.; Tanaka, T.;

Published by: Journal of Geophysical Research: Space Physics Published on: 11/2016

YEAR: 2016 DOI: 10.1002/2016JA023013

injections; nonadiabatic acceleration; substorms; Van Allen Probes

Mesospheric ozone destruction by high-energy electron precipitation associated with pulsating aurora

Energetic particle precipitation into the upper atmosphere creates excess amounts of odd nitrogen and odd hydrogen. These destroy mesospheric and upper stratospheric ozone in catalytic reaction chains, either in situ at the altitude of the energy deposition or indirectly due to transport to other altitudes and latitudes. Recent statistical analysis of satellite data on mesospheric ozone reveals that the variations during energetic electron precipitation from Earth\textquoterights radiation belts can be tens of percent. Here we report model calculations of ozone destruction due to a single event of pulsating aurora early in the morning on 17 November 2012. The presence of high-energy component in the precipitating electron flux (>200 keV) was detected as ionization down to 68 km altitude, by the VHF incoherent scatter radar of European Incoherent Scatter (EISCAT) Scientific Association (EISCAT VHF) in Troms\o, Norway. Observations by the Van Allen Probes satellite B showed the occurrence of rising tone lower band chorus waves, which cause the precipitation. We model the effect of high-energy electron precipitation on ozone concentration using a detailed coupled ion and neutral chemistry model. Due to a 30 min, recorded electron precipitation event we find 14\% odd oxygen depletion at 75 km altitude. The uncertainty of the higher-energy electron fluxes leads to different possible energy deposition estimates during the pulsating aurora event. We find depletion of odd oxygen by several tens of percent, depending on the precipitation characteristics used in modeling. The effect is notably maximized at the sunset time following the occurrence of the precipitation.

Turunen, Esa; Kero, Antti; Verronen, Pekka; Miyoshi, Yoshizumi; Oyama, Shin-Ichiro; Saito, Shinji;

Published by: Journal of Geophysical Research: Atmospheres Published on: 10/2016

YEAR: 2016 DOI: 10.1002/2016JD025015

EISCAT; electron precipitation; ion chemistry; mesosphere; ozone; pulsating aurora; Van Allen Probes

Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

Three mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth\textquoterights outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3 < L* < 5) in less than 6 h after the passage of an interplanetary shock. We model the electron pitch angle distribution under a compressed magnetic field topology based on actual solar wind conditions. Although these ultrarelativistic electrons exhibit highly anisotropic (peaked in 90\textdegree), energy-dependent pitch angle distributions, which appear to be associated with the typical EMIC wave scattering, comparison of the modeled electron distribution to electron measurements indicates that drift shell splitting is responsible for this rapid change in electron pitch angle distributions. This further indicates that magnetopause loss is the predominant cause of the electron dropout right after the shock arrival.

Zhang, X.-J.; Li, W.; Thorne, R.; Angelopoulos, V.; Ma, Q.; Li, J.; Bortnik, J.; Nishimura, Y.; Chen, L.; Baker, D.; Reeves, G.; Spence, H.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Blake, J.; Fennell, J.;

Published by: Journal of Geophysical Research: Space Physics Published on: 09/2016

YEAR: 2016 DOI: 10.1002/2016JA022517

Drift shell splitting; dropouts; magnetic storm; magnetopause shadowing; outer radiation belt; relativistic electron loss; Van Allen Probes

Van Allen Probes Observations of Electromagnetic Ion Cyclotron Waves Triggered by Enhanced Solar Wind Dynamic Pressure

Magnetospheric compression due to impact of enhanced solar wind dynamic pressure Pdyn has long been considered as one of the generation mechanisms of electromagnetic ion cyclotron (EMIC) waves. With the Van Allen Probe-A observations, we identify three EMIC wave events that are triggered by Pdyn enhancements under prolonged northward IMF quiet time preconditions. They are in contrast to one another in a few aspects. Event 1 occurs in the middle of continuously increasing Pdyn while Van Allen Probe-A is located outside the plasmapause at post-midnight and near the equator (magnetic latitude (MLAT) ~ -3o). Event 2 occurs by a sharp Pdyn pulse impact while Van Allen Probe-A is located inside the plasmapause in the dawn sector and rather away from the equator (MLAT ~ 12o). Event 3 is characterized by amplification of a pre-existing EMIC wave by a sharp Pdyn pulse impact while Van Allen Probe-A is located outside the plasmapause at noon and rather away from the equator (MLAT ~ -15o). These three events represent various situations where EMIC waves can be triggered by Pdyn increases. Several common features are also found among the three events. (i) The strongest wave is found just above the He+ gyrofrequency. (ii) The waves are nearly linearly polarized with a rather oblique propagation direction (~28o to ~39o on average). (iii) The proton fluxes increase in immediate response to the Pdyn impact, most significantly in tens of keV energy, corresponding to the proton resonant energy. (iv) The temperature anisotropy with T⊥ > T|| is seen in the resonant energy for all the events, although its increase by the Pdyn impact is not necessarily always significant. The last two points (iii) and (iv) may imply that, in addition to the temperature anisotropy, the increase of the resonant protons must have played a critical role in triggering the EMIC waves by the enhanced Pdyn impact.

Cho, J.-H.; Lee, D.-Y.; Noh, S.-J.; Shin, D.-K.; Hwang, J.; Kim, K.-C.; Lee, J.; Choi, C.; Thaller, S.; Skoug, R.;

Published by: Journal of Geophysical Research: Space Physics Published on: 09/2016

YEAR: 2016 DOI: 10.1002/2016JA022841

Dynamic pressure; EMIC waves; Van Allen Probes

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

Ozaki, M.; Shiokawa, K.; Miyoshi, Y.; Kataoka, R.; Yagitani, S.; Inoue, T.; Ebihara, Y.; Jun, C.-W; Nomura, R.; Sakaguchi, K.; Otsuka, Y.; Shoji, M.; Schofield, I.; Connors, M.; Jordanova, V.;

Published by: Geophysical Research Letters Published on: 08/2016

YEAR: 2016 DOI: 10.1002/2016GL070008

fast modulation; Pc1 geomagnetic pulsations; pulsating proton aurora; subpacket structure; Van Allen Probes; wave-particle interactions

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

Ozaki, M.; Shiokawa, K.; Miyoshi, Y.; Kataoka, R.; Yagitani, S.; Inoue, T.; Ebihara, Y.; Jun, C.-W; Nomura, R.; Sakaguchi, K.; Otsuka, Y.; Shoji, M.; Schofield, I.; Connors, M.; Jordanova, V.;

Published by: Geophysical Research Letters Published on: 08/2016

YEAR: 2016 DOI: 10.1002/2016GL070008

fast modulation; Pc1 geomagnetic pulsations; pulsating proton aurora; subpacket structure; Van Allen Probes; wave-particle interactions

Propagation of ULF waves from the upstream region to the midnight sector of the inner magnetosphere

Ultralow frequency (ULF) waves generated in the ion foreshock are a well-known source of Pc3-Pc4 waves (7\textendash100 mHz) observed in the dayside magnetosphere. We use data acquired on 10 April 2013 by multiple spacecraft to demonstrate that ULF waves of upstream origin can propagate to the midnight sector of the inner magnetosphere. At 1130\textendash1730 UT on the selected day, the two Van Allen Probes spacecraft and the geostationary ETS-VIII satellite detected compressional 20 to 40 mHz magnetic field oscillations between L \~ 4 and L \~ 7 in the midnight sector, along with other spacecraft located closer to noon. Upstream origin of the oscillations is concluded from the wave frequency that matches a theoretical model, globally coherent amplitude modulation, and duskward propagation that is consistent with expected entry of the upstream wave energy through the dawnside flank under the observed interplanetary magnetic field. The oscillations are attributed to magnetohydrodynamic fast-mode waves based on their propagation velocity of \~300 km/s and the relationship between the electric and magnetic field perturbations. The magnitude of the azimuthal wave number is estimated to be \~30. There is no evidence that the oscillations propagated to the ground in the midnight sector.

Takahashi, Kazue; Hartinger, Michael; Malaspina, David; Smith, Charles; Koga, Kiyokazu; Singer, Howard; ühauff, Dennis; Baishev, Dmitry; Moiseev, Alexey; Yoshikawa, Akimasa;

Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016

YEAR: 2016 DOI: 10.1002/2016JA022958

midnight sector; Pc3 waves; plasmasphere; upstream waves; Van Allen Probes

Propagation of ULF waves from the upstream region to the midnight sector of the inner magnetosphere

Takahashi, Kazue; Hartinger, Michael; Malaspina, David; Smith, Charles; Koga, Kiyokazu; Singer, Howard; ühauff, Dennis; Baishev, Dmitry; Moiseev, Alexey; Yoshikawa, Akimasa;

Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016

YEAR: 2016 DOI: 10.1002/2016JA022958

midnight sector; Pc3 waves; plasmasphere; upstream waves; Van Allen Probes

Storm time impulsive enhancements of energetic oxygen due to adiabatic acceleration of preexisting warm oxygen in the inner magnetosphere

We examine enhancements of energetic (>50 keV) oxygen ions observed by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on board the Van Allen Probes spacecraft in the inner magnetosphere (L ~ 6) at 22\textendash23 h magnetic local time (MLT) during an injection event of the 6 June 2013 storm. Simultaneous observations by two Van Allen Probes spacecraft located close together (~0.5 RE) indicate that particle injections occurred in the premidnight sector (< ~24 h MLT). We also examine the evolution of the proton and oxygen energy spectra at L ~ 6 during the injection event. The spectral slope did not significantly change during the storm. The oxygen phase space density (PSD) was shifted toward higher PSD in a wide range of the first adiabatic invariant. The spectral evolution manifests the characteristics of adiabatic acceleration and density increase of oxygen ions. Warm (0.1\textendash10 keV) oxygen measured by the Helium, Oxygen, Proton, and Electron (HOPE) instrument was enhanced prior to the storm mostly in magnetic field-aligned directions. The most reasonable scenario of this event is that warm oxygen ions that preexisted in the inner magnetosphere were picked up and adiabatically transported and accelerated by spatially localized, temporarily impulsive electric fields.

Keika, Kunihiro; Seki, Kanako; e, Masahito; Machida, Shinobu; Miyoshi, Yoshizumi; Lanzerotti, Louis; Mitchell, Donald; Gkioulidou, Matina; Turner, Drew; Spence, Harlan; Larsen, Brian;

Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016

YEAR: 2016 DOI: 10.1002/2016JA022384

adiabatic transport from the plasma sheet; oxygen ions of ionospheric origin; preconditions of magnetic storms; preexisting oxygen ions trapped in the inner magnetosphere; Van Allen Probes; Van Allen Probes RBSPICE observations

ELF/VLF wave propagation at subauroral latitudes: Conjugate observation between the ground and Van Allen Probes A

We report simultaneous observation of ELF/VLF emissions, showing similar spectral and frequency features, between a VLF receiver at Athabasca (ATH), Canada, (L = 4.3) and Van Allen Probes A (Radiation Belt Storm Probes (RBSP) A). Using a statistical database from 1 November 2012 to 31 October 2013, we compared a total of 347 emissions observed on the ground with observations made by RBSP in the magnetosphere. On 25 February 2013, from 12:46 to 13:39 UT in the dawn sector (04\textendash06 magnetic local time (MLT)), we observed a quasiperiodic (QP) emission centered at 4 kHz, and an accompanying short pulse lasting less than a second at 4.8 kHz in the dawn sector (04\textendash06 MLT). RBSP A wave data showed both emissions as right-hand polarized with their Poynting vector earthward to the Northern Hemisphere. Using cross-correlation analysis, we did, for the first time, time delay analysis of a conjugate ELF/VLF event between ground and space, finding +2 to +4 s (ATH first) for the QP and -3 s (RBSP A first) for the pulse. Using backward tracing from ATH to the geomagnetic equator and forward tracing from the equator to RBSP A, based on plasmaspheric density observed by the spacecraft, we validate a possible propagation path for the QP emission which is consistent with the observed time delay.

Martinez-Calderon, Claudia; Shiokawa, Kazuo; Miyoshi, Yoshizumi; Keika, Kunihiro; Ozaki, Mitsunori; Schofield, Ian; Connors, Martin; Kletzing, Craig; Hanzelka, Miroslav; ik, Ondrej; Kurth, William;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2016

YEAR: 2016 DOI: 10.1002/jgra.v121.610.1002/2015JA022264

conjugate event; propagation; QP; Ray Tracing; time delay; Van Allen Probes; VLF/ELF

ELF/VLF wave propagation at subauroral latitudes: Conjugate observation between the ground and Van Allen Probes A

Martinez-Calderon, Claudia; Shiokawa, Kazuo; Miyoshi, Yoshizumi; Keika, Kunihiro; Ozaki, Mitsunori; Schofield, Ian; Connors, Martin; Kletzing, Craig; Hanzelka, Miroslav; ik, Ondrej; Kurth, William;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2016

YEAR: 2016 DOI: 10.1002/jgra.v121.610.1002/2015JA022264

conjugate event; propagation; QP; Ray Tracing; time delay; Van Allen Probes; VLF/ELF

Observations of the impenetrable barrier, the plasmapause, and the VLF bubble during the 17 March 2015 storm

Van Allen Probes observations during the 17 March 2015 major geomagnetic storm strongly suggest that VLF transmitter-induced waves play an important role in sculpting the earthward extent of outer zone MeV electrons. A magnetically confined bubble of very low frequency (VLF) wave emissions of terrestrial, human-produced origin surrounds the Earth. The outer limit of the VLF bubble closely matches the position of an apparent barrier to the inward extent of multi-MeV radiation belt electrons near 2.8 Earth radii. When the VLF transmitter signals extend beyond the eroded plasmapause, electron loss processes set up near the outer extent of the VLF bubble create an earthward limit to the region of local acceleration near L = 2.8 as MeV electrons are scattered into the atmospheric loss cone.

Foster, J.; Erickson, P.; Baker, D.; Jaynes, A.; Mishin, E.; Fennel, J.; Li, X.; Henderson, M.; Kanekal, S.;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2016

YEAR: 2016 DOI: 10.1002/jgra.v121.610.1002/2016JA022509

barrier; Plasmapause; storm; Van Allen Probes; VLF

Rapid enhancement of low-energy (<100 eV) ion flux in response to interplanetary shocks based on two Van Allen Probes case studies: Implications for source regions and heating mechanisms

Interactions between interplanetary (IP) shocks and the Earth\textquoterights magnetosphere manifest many important space physics phenomena including low-energy ion flux enhancements and particle acceleration. In order to investigate the mechanisms driving shock-induced enhancement of low-energy ion flux, we have examined two IP shock events that occurred when the Van Allen Probes were located near the equator while ionospheric and ground observations were available around the spacecraft footprints. We have found that, associated with the shock arrival, electromagnetic fields intensified, and low-energy ion fluxes, including H+, He+, and O+, were enhanced dramatically in both the parallel and perpendicular directions. During the 2 October 2013 shock event, both parallel and perpendicular flux enhancements lasted more than 20 min with larger fluxes observed in the perpendicular direction. In contrast, for the 15 March 2013 shock event, the low-energy perpendicular ion fluxes increased only in the first 5 min during an impulse of electric field, while the parallel flux enhancement lasted more than 30 min. In addition, ionospheric outflows were observed after shock arrivals. From a simple particle motion calculation, we found that the rapid response of low-energy ions is due to drifts of plasmaspheric population by the enhanced electric field. However, the fast acceleration in the perpendicular direction cannot solely be explained by E \texttimes B drift but betatron acceleration also plays a role. Adiabatic acceleration may also explain the fast response of the enhanced parallel ion fluxes, while ion outflows may contribute to the enhanced parallel fluxes that last longer than the perpendicular fluxes.

Yue, Chao; Li, Wen; Nishimura, Yukitoshi; Zong, Qiugang; Ma, Qianli; Bortnik, Jacob; Thorne, Richard; Reeves, Geoffrey; Spence, Harlan; Kletzing, Craig; Wygant, John; Nicolls, Michael;

Published by: Journal of Geophysical Research: Space Physics Published on: 06/2016

YEAR: 2016 DOI: 10.1002/2016JA022808

adiabatic accelerations; enhancement of low-energy ion flux; ionospheric ion outflows; response to IP shocks; Van Allen Probes

Combined Scattering Loss of Radiation Belt Relativistic Electrons by Simultaneous Three-band EMIC Waves: A Case Study

Multiband electromagnetic ion cyclotron (EMIC) waves can drive efficient scattering loss of radiation belt relativistic electrons. However, it is statistically uncommon to capture the three bands of EMIC waves concurrently. Utilizing data from the Electric and Magnetic Field Instrument Suite and Integrated Science magnetometer onboard Van Allen Probe A, we report the simultaneous presence of three (H+, He+, and O+) emission bands in an EMIC wave event, which provides an opportunity to look into the combined scattering effect of all EMIC emissions and the relative roles of each band in diffusing radiation belt relativistic electrons under realistic circumstances. Our quantitative results, obtained by quasi-linear diffusion rate computations and 1-D pure pitch angle diffusion simulations, demonstrate that the combined resonant scattering by the simultaneous three-band EMIC waves is overall dominated by He+ band wave diffusion, mainly due to its dominance over the wave power (the mean wave amplitudes are approximately 0.4 nT, 1.6 nT, and 0.15 nT for H+, He+, and O+ bands, respectively). Near the loss cone, while 2\textendash3 MeV electrons undergo pitch angle scattering at a rate of the order of 10-6\textendash10-5 s-1, 5\textendash10 MeV electrons can be diffused more efficiently at a rate of the order of 10-3\textendash10-2 s-1, which approaches the strong diffusion level and results in a moderately or heavily filled loss cone for the atmospheric loss. The corresponding electron loss timescales (i.e., lifetimes) vary from several days at the energies of ~2 MeV to less than 1 h at ~10 MeV. This case study indicates the leading contribution of He+ band waves to radiation belt relativistic electron losses during the coexistence of three EMIC wave bands and suggests that the roles of different EMIC wave bands in the relativistic electron dynamics should be carefully incorporated in future modeling efforts.

He, Fengming; Cao, Xing; Ni, Binbin; Xiang, Zheng; Zhou, Chen; Gu, Xudong; Zhao, Zhengyu; Shi, Run; Wang, Qi;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016

YEAR: 2016 DOI: 10.1002/2016JA022483

combined scattering rates; electromagnetic ion cyclotron waves; loss timescales; radiation belt relativistic electrons; resonant wave-particle interactions; Van Allen Probes

Simulation of energy-dependent electron diffusion processes in the Earth\textquoterights outer radiation belt

The radial and local diffusion processes induced by various plasma waves govern the highly energetic electron dynamics in the Earth\textquoterights radiation belts, causing distinct characteristics in electron distributions at various energies. In this study, we present our simulation results of the energetic electron evolution during a geomagnetic storm using the University of California, Los Angeles 3-D diffusion code. Following the plasma sheet electron injections, the electrons at different energy bands detected by the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT) instruments on board the Van Allen Probes exhibit a rapid enhancement followed by a slow diffusive movement in differential energy fluxes, and the radial extent to which electrons can penetrate into depends on energy with closer penetration toward the Earth at lower energies than higher energies. We incorporate radial diffusion, local acceleration, and loss processes due to whistler mode wave observations to perform a 3-D diffusion simulation. Our simulation results demonstrate that chorus waves cause electron flux increase by more than 1 order of magnitude during the first 18 h, and the subsequent radial extents of the energetic electrons during the storm recovery phase are determined by the coupled radial diffusion and the pitch angle scattering by EMIC waves and plasmaspheric hiss. The radial diffusion caused by ULF waves and local plasma wave scattering are energy dependent, which lead to the observed electron flux variations with energy dependences. This study suggests that plasma wave distributions in the inner magnetosphere are crucial for the energy-dependent intrusions of several hundred keV to several MeV electrons.

Ma, Q.; Li, W.; Thorne, R.; Nishimura, Y.; Zhang, X.-J.; Reeves, G.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Henderson, M.; Spence, H.; Baker, D.; Blake, J.; Fennell, J.; Angelopoulos, V.;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016

YEAR: 2016 DOI: 10.1002/2016JA022507

electron acceleration and loss; energy-dependent diffusion; radial diffusion; radiation belt simulation; Van Allen Probes

A statistical study of proton pitch angle distributions measured by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE)

A statistical study of ring current-energy proton pitch angle distributions (PADs) in Earth\textquoterights inner magnetosphere is reported here. The data are from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the Van Allen Probe B spacecraft from January 1, 2013 to April 15, 2015. By fitting the data to the functional form sinnα, where α is the proton pitch angle, we examine proton PADs at the energies 50, 100, 180, 328 and 488 keV in the L-shell range from L = 2.5 to L = 6. Three PAD types are classified: trapped (90\textdegree peaked), butterfly and isotropic. The proton PAD dependence on the particle energy, MLT, L-shell, and geomagnetic activity are analyzed in detail. The results show a strong dependence of the proton PADs on MLT. On the nightside, the n values outside the plasmapause are clearly lower than those inside the plasmapause. At higher energies and during intense magnetic activity, nightside butterfly PADs can be observed at L-shells down to the vicinity of the plasmapause. The averaged n values on the dayside are larger than on the nightside. A maximum of the averagedn values occurs around L = 4.5 in the postnoon sector (12 - 16MLT). The averaged n values show a dawn-dusk asymmetry with lower values on the dawnside at high L-shells, which is consistent with previous studies of butterfly PADs. The MLT dependence of the proton PADs becomes more distinct with increasing particle energy. These features suggest that drift-shell splitting coupled with a radial flux gradient play an important role in the formation of PADs, particularly at L > ~ 4.5

Shi, Run; Summers, Danny; Ni, Binbin; Manweiler, Jerry; Mitchell, Donald; Lanzerotti, Louis;

Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016

YEAR: 2016 DOI: 10.1002/2015JA022140

proton pitch angle distributions; Van Allen Probes

Pulsating proton aurora caused by rising tone Pc1 waves

We found rising tone emissions with a dispersion of \~1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over \~30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7\textdegreeN, 246.7\textdegreeE, magnetic latitude 61.7\textdegreeN). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, \~10\% variations in intensity, and fine structures of 3\textdegree in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of \~1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.;

Published by: Journal of Geophysical Research: Space Physics Published on: 02/2016

YEAR: 2016 DOI: 10.1002/2015JA021681

EMIC-triggered waves; Pc1 waves; proton aurora

Pulsating proton aurora caused by rising tone Pc1 waves

Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.;

Published by: Journal of Geophysical Research: Space Physics Published on: 02/2016

YEAR: 2016 DOI: 10.1002/2015JA021681

EMIC-triggered waves; Pc1 waves; proton aurora

Relativistic electron microbursts and variations in trapped MeV electron fluxes during the 8-9 October 2012 storm: SAMPEX and Van Allen Probes observations

It has been suggested that whistler mode chorus is responsible for both acceleration of MeV electrons and relativistic electron microbursts through resonant wave-particle interactions. Relativistic electron microbursts have been considered as an important loss mechanism of radiation belt electrons. Here we report on the observations of relativistic electron microbursts and flux variations of trapped MeV electrons during the 8\textendash9 October 2012 storm, using the SAMPEX and Van Allen Probes satellites. Observations by the satellites show that relativistic electron microbursts correlate well with the rapid enhancement of trapped MeV electron fluxes by chorus wave-particle interactions, indicating that acceleration by chorus is much more efficient than losses by microbursts during the storm. It is also revealed that the strong chorus wave activity without relativistic electron microbursts does not lead to significant flux variations of relativistic electrons. Thus, effective acceleration of relativistic electrons is caused by chorus that can cause relativistic electron microbursts.

Kurita, Satoshi; Miyoshi, Yoshizumi; Blake, Bernard; Reeves, Geoffery; Kletzing, Craig;

Published by: Geophysical Research Letters Published on: 02/2016

YEAR: 2016 DOI: 10.1002/2016GL068260

Radiation belts; relativistic electron microbursts; relativistic electrons; SAMPEX; Van Allen Probes; whistler mode chorus

2015

Survey of radiation belt energetic electron pitch angle distributions based on the Van Allen Probes MagEIS measurements

A statistical survey of electron pitch angle distributions (PADs) is performed based on the pitch angle resolved flux observations from the Magnetic Electron Ion Spectrometer (MagEIS) instrument on board the Van Allen Probes during the period from 1 October 2012 to 1 May 2015. By fitting the measured PADs to a sinnα form, where α is the local pitch angle and n is the power law index, we investigate the dependence of PADs on electron kinetic energy, magnetic local time (MLT), the geomagnetic Kp index and L-shell. The difference in electron PADs between the inner and outer belt is distinct. In the outer belt, the common averaged n values are less than 1.5, except for large values of the Kp index and high electron energies. The averaged n values vary considerably with MLT, with a peak in the afternoon sector and an increase with increasing L-shell. In the inner belt, the averaged n values are much larger, with a common value greater than 2. The PADs show a slight dependence on MLT, with a weak maximum at noon. A distinct region with steep PADs lies in the outer edge of the inner belt where the electron flux is relatively low. The distance between the inner and outer belt and the intensity of the geomagnetic activity together determine the variation of PADs in the inner belt. Besides being dependent on electron energy, magnetic activity and L-shell, the results show a clear dependence on MLT, with higher n values on the dayside.

Shi, Run; Summers, Danny; Ni, Binbin; Fennell, Joseph; Blake, Bernard; Spence, Harlan; Reeves, Geoffrey;

Published by: Journal of Geophysical Research: Space Physics Published on: 12/2015

YEAR: 2015 DOI: 10.1002/2015JA021724

pitch angle distributions; Van Allen Probes

Formation process of relativistic electron flux through interaction with chorus emissions in the Earth\textquoterights inner magnetosphere

We perform test particle simulations of energetic electrons interacting with whistler mode chorus emissions. We compute trajectories of a large number of electrons forming a delta function with the same energy and equatorial pitch angle. The electrons are launched at different locations along the magnetic field line and different timings with respect to a pair of chorus emissions generated at the magnetic equator. We follow the evolution of the delta function and obtain a distribution function in energy and equatorial pitch angle, which is a numerical Green\textquoterights function for one cycle of chorus wave-particle interaction. We obtain the Green\textquoterights functions for the energy range 10 keV\textendash6 MeV and all pitch angles greater than the loss cone angle. By taking the convolution integral of the Green\textquoterights functions with the distribution function of the injected electrons repeatedly, we follow a long-time evolution of the distribution function. We find that the energetic electrons are accelerated effectively by relativistic turning acceleration and ultrarelativistic acceleration through nonlinear trapping by chorus emissions. Further, these processes result in the rapid formation of a dumbbell distribution of highly relativistic electrons within a few minutes after the onset of the continuous injection of 10\textendash30 keV electrons.

Omura, Yoshiharu; Miyashita, Yu; Yoshikawa, Masato; Summers, Danny; Hikishima, Mitsuru; Ebihara, Yusuke; Kubota, Yuko;

Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015

YEAR: 2015 DOI: 10.1002/2015JA021563

Chorus; nonlinear wave-particle interaction; Particle acceleration; Radiation belts; relativistic electrons; simulation

Formation process of relativistic electron flux through interaction with chorus emissions in the Earth\textquoterights inner magnetosphere

Omura, Yoshiharu; Miyashita, Yu; Yoshikawa, Masato; Summers, Danny; Hikishima, Mitsuru; Ebihara, Yusuke; Kubota, Yuko;

Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015

YEAR: 2015 DOI: 10.1002/2015JA021563

Chorus; nonlinear wave-particle interaction; Particle acceleration; Radiation belts; relativistic electrons; simulation

Formation process of relativistic electron flux through interaction with chorus emissions in the Earth\textquoterights inner magnetosphere

Omura, Yoshiharu; Miyashita, Yu; Yoshikawa, Masato; Summers, Danny; Hikishima, Mitsuru; Ebihara, Yusuke; Kubota, Yuko;

Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015

YEAR: 2015 DOI: 10.1002/2015JA021563

Chorus; nonlinear wave-particle interaction; Particle acceleration; Radiation belts; relativistic electrons; simulation

Kinetic Alfv\ en Waves and Particle Response Associated with a Shock-Induced, Global ULF Perturbation of the Terrestrial Magnetosphere

On 2 October 2013, the arrival of an interplanetary shock compressed the Earth\textquoterights magnetosphere and triggered a global ULF (ultra low frequency) oscillation. The Van Allen Probe B spacecraft observed this large-amplitude ULF wave in situ with both magnetic and electric field data. Broadband waves up to approximately 100 Hz were observed in conjunction with, and modulated by, this ULF wave. Detailed analysis of fields and particle data reveals that these broadband waves are Doppler-shifted kinetic Alfv\ en waves. This event suggests that magnetospheric compression by interplanetary shocks can induce abrupt generation of kinetic Alfv\ en waves over large portions of the inner magnetosphere, potentially driving previously unconsidered wave-particle interactions throughout the inner magnetosphere during the initial response of the magnetosphere to shock impacts.

Malaspina, David; Claudepierre, Seth; Takahashi, Kazue; Jaynes, Allison; Elkington, Scot; Ergun, Robert; Wygant, John; Reeves, Geoff; Kletzing, Craig;

Published by: Geophysical Research Letters Published on: 11/2015

YEAR: 2015 DOI: 10.1002/2015GL065935

inner magnetosphere; interplanetary shock; Kinetic Alfven Waves; magnetosphere shock response; plasma waves; ULF waves; Van Allen Probes

Multifrequency compressional magnetic field oscillations and their relation to multiharmonic toroidal mode standing Alfv\ en waves

The power spectrum of the compressional component of magnetic fields observed by the Van Allen Probes spacecraft near the magnetospheric equator in the dayside plasmasphere sometimes exhibits regularly spaced multiple peaks at frequencies below 50 mHz. We show by detailed analysis of events observed on two separate days in early 2014 that the frequencies change smoothly with the radial distance of the spacecraft and appear at or very near the frequencies of the odd harmonics of mutiharmonic toroidal mode standing Alfv\ en waves seen in the azimuthal component of the magnetic field. Even though the compressional component had a low amplitude on one of the selected days, its spectral properties are highlighted by computing the ratio of the spectral powers of time series data obtained from two spatially separated Van Allen Probes spacecraft. The spectral similarity of the compressional and azimuthal components suggests that the compressional component contains field line resonance characteristics.

Takahashi, Kazue; Waters, Colin; Glassmeier, Karl-Heinz; Kletzing, Craig; Kurth, William; Smith, Charles;

Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015

YEAR: 2015 DOI: 10.1002/2015JA021780

Compressional oscillations; Field line resonance; Pc3-Pc4 band; plasmasphere; Van Allen Probes

On the formation and origin of substorm growth phase/onset auroral arcs inferred from conjugate space-ground observations

Magnetotail processes and structures related to substorm growth phase/onset auroral arcs remain poorly understood mostly due to the lack of adequate observations. In this study we make a comparison between ground-based optical measurements of the premidnight growth phase/onset arcs at subauroral latitudes and magnetically conjugate measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) at ~780 km in altitude and by the Van Allen Probe B (RBSP-B) spacecraft crossing L values of ~5.0\textendash5.6 in the premidnight inner tail region. The conjugate observations offer a unique opportunity to examine the detailed features of the arc location relative to large-scale Birkeland currents and of the magnetospheric counterpart. Our main findings include (1) at the early stage of the growth phase the quiet auroral arc emerged ~4.3\textdegree equatorward of the boundary between the downward Region 2 (R2) and upward Region 1 (R1) currents; (2) shortly before the auroral breakup (poleward auroral expansion) the latitudinal separation between the arc and the R1/R2 demarcation narrowed to ~1.0\textdegree; (3) RBSP-B observed a magnetic field signature of a local upward field-aligned current (FAC) connecting the arc with the near-Earth tail when the spacecraft footprint was very close to the arc; and (4) the upward FAC signature was located on the tailward side of a local plasma pressure increase confined near L ~5.2\textendash5.4. These findings strongly suggest that the premidnight arc is connected to highly localized pressure gradients embedded in the near-tail R2 source region via the local upward FAC.

Motoba, T.; Ohtani, S.; Anderson, B.; Korth, H.; Mitchell, D.; Lanzerotti, L.; Shiokawa, K.; Connors, M.; Kletzing, C.; Reeves, G.;

Published by: Journal of Geophysical Research: Space Physics Published on: 10/2015

YEAR: 2015 DOI: 10.1002/jgra.v120.1010.1002/2015JA021676

FACs; growth phase/onset arc; M-I coupling; Van Allen Probes

Giant pulsations on the afternoonside: Geostationary satellite and ground observations

Giant pulsations (Pgs) are a special class of oscillations recognized in ground magnetometer records as exhibiting highly regular sinusoidal waveforms in the east-west component with periods around 100s. Previous statistical studies showed that Pgs occur almost exclusively on the morningside with peak occurrence in the postmidnight sector. In this paper, we present observations of Pgs extending to the afternoonside, using data from the GOES13 and 15 geostationary satellites and multiple ground magnetometers located in North America. For a long-lasting event on 29 February 2012, which spanned \~08\textendash18h magnetic local time, we show that basic Pg properties did not change with the local time, although the period of the pulsations was longer at later local time due to increasing mass loading. There is evidence that the Pgs resulted from fundamental poloidal mode standing Alfv\ en waves, both on the morning and afternoonsides. Oscillations of energetic particles associated with the field oscillations exhibited an energy-dependent phase, which has previously been reported and explained by drift resonance. A statistical analysis of the ground magnetic field data (L = 3.8\textendash7.4) covering 2008\textendash2013 confirms that afternoon Pgs are not unusual. We identified a total of 105 Pg events (about 70\% (30\%) of the events occurred during non-storm (late storm recovery) periods), 31 of which occurred on the afternoonside. The afternoon Pgs occur under solar wind and geomagnetic conditions that are similar to the morning Pgs, but the afternoon Pgs tend to have short durations and occur frequently in winter instead of around spring and fall equinoxes that are favored by the morning Pgs.

Motoba, Tetsuo; Takahashi, Kazue; Rodriguez, Juan; Russell, Christopher;

Published by: Journal of Geophysical Research: Space Physics Published on: 10/2015

YEAR: 2015 DOI: 10.1002/2015JA021592

giant pulsations; ground-space conjunction; wave-particle interactions

Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave-induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H+-band EMIC waves dominate the scattering losses of ~1\textendash4 MeV outer zone relativistic electrons, it is He+-band and O+-band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave-induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi-parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H+-band EMIC wave scattering rates at pitch angles < ~40\textdegree for electrons > ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from <1 min to ~1 h. The electron loss cone can be either empty as a result of the weak diffusion or heavily/fully filled due to approaching the strong diffusion limit, while the trapped electron population at high pitch angles close to 90\textdegree remains intact because of no resonant scattering. In this manner, EMIC wave scattering has the potential to deepen the anisotropic distribution of outer zone relativistic electrons by reshaping their pitch angle profiles to \textquotedbllefttop-hat.\textquotedblright Overall, H+-band and He+-band EMIC waves are most efficient in producing the pitch angle scattering loss of relativistic electrons at ~1\textendash2 MeV. In contrast, the presence of O+-band EMIC waves, while at a smaller occurrence rate, can dominate the scattering loss of 5\textendash10 MeV electrons in the entire region of the outer zone, which should be considered in future modeling of the outer zone relativistic electron dynamics.

Ni, Binbin; Cao, Xing; Zou, Zhengyang; Zhou, Chen; Gu, Xudong; Bortnik, Jacob; Zhang, Jichun; Fu, Song; Zhao, Zhengyu; Shi, Run; Xie, Lun;

Published by: Journal of Geophysical Research: Space Physics Published on: 09/2015

YEAR: 2015 DOI: 10.1002/2015JA021466

electron loss time scales; EMIC waves; outer radiation belt; relativistic electrons; resonant wave-particle interactions

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